LED Alternatives for Fluorescent

The Big Difference

• Fluorescent bulbs are filled with an ionized gas containing a small percentage of mercury vapor. When electric current runs through the gas, the mercury absorbs some of that energy and emits it as ultraviolet light. The ultraviolet light hits against a phosphor coating on the inside of the bulb. The phosphor absorbs the invisible ultraviolet light and emits visible light.

  LEDs make their light by adding energy to electrons in a semiconductor chip. The amount of energy is determined by the material and fabrication of the LED, but each LED emits a single wavelength. To make white light, either three (or more) different color LEDs are put together, or a blue LED is used to excite a yellow-red phosphor.

  The differences in the fundamental processes that create light lead to performance differences between fluorescent and LED illumination.

Efficacy

• Efficacy is a measure of how well a bulb converts electrical power to visible light. It's measured in lumens per watt. Lumens are a measure of the optical power contained in the visible portion of the emitted light. Compact fluorescents have an efficacy of around 60 to 70 lumens per watt, while the 4-foot T5 and T8 bulbs are up around 80 to 100 lumens per watt. Although LEDs are a pretty new technology, they can have efficacies even higher than 100 lumens per watt, but there is a wide variation, with some efficacies down in the 50 lumen per watt range.

Color Rendering Index

• Color is a complex subject, because it depends not just on the quality of the light source, but also the color of specific objects and the perceptual mechanisms within the brain. Color Rendering Index (CRI) is a metric for comparing the quality of white light sources. Although there is some controversy about the quality of the CRI itself, it's the best metric out there right now. A value of 100 means the light source makes colors appear as they do under bright sunlight.

  Modern fluorescent tubes have a CRI of about 85. LEDs designed to replace fluorescent tubes vary in CRI from 56 to 89.

Color Temperature

• When the heating element in a toaster turns on, it begins glowing a dull red, moves to a brighter red, then to orange, and perhaps even to yellow. The color depends on the temperature. The sun, for example, emits its yellow light because it's at a temperature of 5000 K (about 8500°F). Artificial light sources are characterized by the correlated color temperature, which indicates what temperature the source would be at if it were a simple heated object. For incandescent bulbs, this is the same as their actual temperature, around 2300K. Fluorescents and LEDs make their light differently, so its a bit more complicated, but the correlated color temperature is measured, and can be used to compare sources.

  Compact fluorescents, for example, can vary in color temperature from 2300 K to 6500 K. LEDs formatted for those same sockets vary in color temperature from 2700 K to more than 7100 K.

A General Rule

• LED bulbs are generally at least as efficient as fluorescents, but their total light output does not quite reach the total possible with fluorescents. For compact fluorescents, LEDs are comparable to the 600- to 800-lumen bulbs available, but compared with fluorescent tubes, LEDs provide about half as much light. For some applications the other advantages of LEDs, particularly the longer lifetime, outweigh the reduced total light output.

Catalog of Specific Bulbs

• The Department of Energy maintains a catalog of light source performance for specific bulbs, both fluorescent and LED, available at the Energy Star and Lighting Facts websites. Using those listings, illumination characteristics of specific bulbs can be compared, including the efficacy, lumen output, CRI, and color temperature.